1. Field of the Invention
The present invention relates to dielectric filters, dielectric duplexers, and communication apparatuss incorporating the same, which are used in high-frequency bands.
2. Description of the Related Art
FIGS. 10A to 10E show the structure of a dielectric filter using a dielectric block, which is mainly used in a micro-wave band. FIG. 10B is a front view of the dielectric filter stood up, FIG. 10A is a top view thereof, FIG. 10C is a bottom view thereof, FIG. 10D is a left-side view thereof, and FIG. 10E is a right-side view thereof. In FIGS. 10A to 10E, a reference numeral 1 denotes a dielectric block. Inside the dielectric block 1, resonance line holes indicated by reference numerals 2a, 2b, and 2c are formed. On the inner surfaces of the resonance line holes, inner conductors are disposed to form resonance lines 5a, 5b and 5c. A ground electrode 3 is formed on an external surface of the dielectric block 1, and external terminals 6 and 7 are provided by insulating from the ground electrode 3. The external terminal 6 capacitively couples with the resonance line 5a, and the external terminal 7 capacitively couples with the resonance line 5c. In this way, a dielectric filter having band pass characteristics of a three-stage resonator is constituted.
In such a dielectric filter shown in FIGS. 10A to 10E, the external terminals 6 and 7 performs an unbalanced-type input/output of signals while using each ground electrode as a reference potential. In order to send a signal to a balanced-input-type amplifying circuit, for example, a balun (an unbalance-balance conversion unit) must be used to convert an unbalanced-type signal into a balanced-type signal. As a result, the area occupied by a filter-circuit part on a circuit board is increased, which leads to a hindrance to miniaturization.
To overcome the above described problems, preferred embodiments of the present invention provide a dielectric filter, a dielectric duplexer, and a communication apparatus incorporating the same, in which the balanced-type input/output of signals can be performed without using a balun mentioned above.
One preferred embodiment of the present invention provides a dielectric filter comprising: a xcex/2 resonator for generating resonance of xc2xd-wavelength at a predetermined frequency, having both ends open-circuited or short-circuited; and a pair of xcex/4 resonators respectively for generating resonance of xc2xc-wavelength at a frequency substantially equal to the predetermined frequency, each having one end open-circuited and the other end short-circuited; wherein the pair of xcex/4 resonators are disposed in proximity to each of both ends from the vicinity of the center of the xcex/2 resonator; a terminal coupling to the xcex/2 resonator is provided as an unbalanced terminal; and terminals coupling to the pair of xcex/4 resonators is used as a balanced terminal.
According to the above structure and arrangement, an unbalanced terminal and balanced terminals can be used to input and output signals, and pass and attenuation in a predetermined frequency band can also be performed by using these terminals.
In the above described dielectric filter, the xcex/2 resonator may be bent at substantially the center of the xcex/2 resonator.
According to the above described arrangement, a xcex/2 resonator and xcex/4 resonators coupling thereto can be disposed at both sides, by which a compact arrangement can be obtained in a restricted space.
Another preferred embodiment of the present invention provides a dielectric filter comprising: a first xcex/2 resonator for generating resonance of xc2xd-wavelength at a predetermined frequency, having both ends open-circuited or short-circuited; and a second xcex/2 resonator for generating resonance of xc2xd-wavelength at a frequency substantially equal to the predetermined frequency, having both ends open-circuited; wherein the second xcex/2 resonator is disposed in proximity to the first xcex/2 resonator; a terminal coupling to the first xcex/2 resonator is provided as an unbalanced terminal; and two terminals coupling to the second xcex/2 resonator are provided as balanced terminals.
According to the above structure and arrangement, an unbalanced terminal and balanced terminals can be used to input and output signals, and pass and attenuation in a predetermined frequency band can also be performed by using these terminals as well.
In the above described dielectric filter, the xcex/2 resonator and the xcex/4 resonator may be each either formed by a micro stripline or a stripline.
According to the above structure and arrangement, without disposing a balun, in addition to a circuit for performing the balanced input/output of signals and a circuit for performing the unbalanced input/output of signals, a circuit having a filter can be easily formed on a dielectric substrate.
In the above described dielectric filter, the xcex/2 resonator and the xcex/4 resonator may be formed by a dielectric coaxial resonator comprising a dielectric block on which a conductor film is disposed.
According to the above structure and arrangement, although the dielectric filter has a coaxial resonator, when the dielectric filter is only mounted on a printed circuit board or the like, in addition to a circuit for performing the balanced input/output of signals and a circuit for performing the unbalanced input/output of signals, a circuit having a filter can be easily formed without the need for a balun.
Yet another preferred embodiment of the present invention provides a dielectric duplexer comprising the dielectric filter described above.
Yet another preferred embodiment of the present invention provides a communication apparatus comprising the dielectric filter or the dielectric duplexer described above.
The above described communication apparatus can be formed in a compact size with lightweight.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.